Add low-level function to list existing remotes on Git repository
Rename remotes() to get_merge_remotes() and add check in rcleanup()

Ensure merge_remotes exist locally before attempting to remove them
Add decorator for GH API calls retries

Recent usages of scc have been failing with github.GithubException 502 Server
Error. This commit adds a decorator implementing retries if a server error is
thrown. Fixes #114.
Decorate __getattr__ methods of various GH objects with retry_on_error
Decorate create_instance() method of GHManager
Merge pull request #116 from sbesson/114_retry_decorator

Add decorator for GH API calls retries
Merge pull request #117 from sbesson/75_rcleanup

Check for remote existence in rcleanup